I have reviewed this document as part of the security directorate's ongoing effort to review all IETF documents being processed by the IESG. These comments were written primarily for the benefit of the security area directors. Document editors and WG chairs should treat these comments just like any other last-call comments.

This document defines an algorithm (HyStart++) for the start phase of congestion control to prevent high packet loss and jitter. While the algorithm is intended for TCP, it can also be used by QUIC and SCTP.

Nits:
Please expand RTO on first use.

The "Security Considerations" section only contains a pointer to RFC 5681. I think this is insufficient. I recommend copying the text from RFC 5681 that is applicable while retaining a reference to RFC 5681. For example, it is not clear to me if the RECOMMENDATION against ACK division attack stated in RFC 5681 is also applicable for HyStart++.